FIGS. 5(a)-5(d) show sequential steps followed in forming a fine pattern by conventional semiconductor lithography technology. First, a hydrophobic treatment is applied to a semiconductor substrate 51 before an organic or inorganic thin film is coated on the substrate to form a resist. The hydrophobic treatment improves the adhesion of the resist to the substrate. Resist 52 is typically deposited by a thin-film coating method. Following the coating of the resist 52 on substrate 51, the substrate is subjected to a heat treatment to remove any solvent contained in the resist. Hereinafter this step is referred to as a baking step. For example, the baking step is performed by placing the substrate on a hot plate set at a predetermined temperature for a predetermined amount of time.
As shown in FIG. 5(b) after baking, the resist is exposed to energy beams 53, such as ultra-violet rays or electron beams, in accordance with a desired pattern to form a latent image 54 of the pattern causing a chemical change to take place in the exposed area of resist 52. The substrate is then dipped into a developer causing a chemical change to take place in the exposed area of resist 52, and due to a difference of dissolution velocity with respect to the developer for the exposed and unexposed parts of the resist, the desired pattern is formed. When the dissolution velocity of latent image portion 54 is low, the exposed portion remains and a negative type resist pattern is formed as shown in FIG. 5(c). On the other hand, when the dissolution velocity of the latent image portion 54 increases or is high, the exposed portion is dissolved and a positive type resist pattern is obtained as shown in FIG. 5(d). After forming the pattern in this way, the substrate is further processed by dry etching and selective introduction of impurity region(s) by ion implantation, for example.
As the size of the pattern to be formed becomes smaller and smaller, the dose of the energy beams required for exposure increases. That is, when the pattern to be formed has a relatively large size, a point on the resist receives a high amount of energy during exposure due to the influence of adjacent exposed areas resulting from scattering of the energy beams, and so forth. Therefore, a low dosage of the exposure beam is sufficient. On the other hand, when the formation pattern is of a small size, an exposure point on the resist is not as easily affected by adjacent exposed areas and therefore its energy does not increase. Accordingly, the dosage of exposure beams must be increased. Therefore, with the formation of smaller or finer patterns, the dosage of energy required to obtain a predetermined degree of exposure per unit area increases. To solve this problem, the sensitivity of the resist has been increased, as disclosed, for example, in "chemical amplification resist", Journal of Vacuum Science and Technology, V. B6, 379-383 (1988).
For forming a negative type resist, the exposed portions of which remain at the time of development, a skeleton-forming novolak resin is used with an acid generator and a cross linker. The acid generator contained in the exposed portion of the resist generates an acid (hydrogen ion) that functions as a catalyst at the time of baking (after exposure) to start a cross linking reaction of the novolak resin. Accordingly, the exposed portions of the novolak resin become polymeric and the solubility of them in the developer drops remarkably. Thus, the exposed pattern portions remain after development to provide the negative type pattern that is desired.
For a positive type resist, a dissolution inhibitor is used in place of the cross linker for suppressing dissolution of the novolak resin in the developer. The acid generated during exposure acts on the dissolution inhibitor during baking and functions as a catalyst to reduce the dissolution inhibiting effect by the decomposition reaction of the dissolution inhibitor. As a result, the surrounding novolak resin is dissolved in the developer and a positive type pattern is obtained.
During exposure, the cross linker or the dissolution inhibitor directly receives the energy of the exposure beams and the reaction proceeds accordingly. In the chemical amplification resist, however, energy is imparted to the acid generator. The pattern is obtained if the acid is generated and the catalytic reaction proceeds. It is known that the acid can be generated from the acid generator by applying low energy exposure of the resist. Accordingly, the chemical amplification resist has a possibility of high sensitivity to exposure.
By using a chemical amplification resist, a fine pattern of about 0.1 .mu.m (100 nm) can be obtained with a low dosage of exposure energy beams. As described, for example in Japanese Journal of Applied Physics 30, 2377-3281 (1991), a fine pattern of the 0.1 .mu.m level is formed using SAL 601-ER7 (a trade name of the Shipley Company). This resist is known as a negative type electron beam chemical amplification resist that is used for device fabrication.